The present invention relates to a fixing apparatus used for an image forming apparatus such as a copier, printer, or facsimile, and particularly to a fixing apparatus having a heat rotating body such as a heat roller or heat fixing belt, and a pressing rotating body which pressure-contacts with the heat rotating body, and to an image forming apparatus provided with the fixing apparatus.
Conventionally, in an image forming apparatus such as the copier, printer, or facsimile, in the fixing apparatus for heat-fixing process of a recorded material carrying a toner formed of heat-fusible resin, the heat roller system are frequently adopted.
The heat roller system fixing apparatus is composed of a metallic heat roller provided with a heat generation source such as a halogen heater therein, and an resilient pressure roller to pressure-contact with it, and when a recorded material such as paper sheet is passed through a fixing nip portion which is a pressure-contact portion of this pair of rollers, a toner image carried on the recorded material is heated and fixed.
In the fixing apparatus which is mounted in an electrophotographic system image forming apparatus to output a color image, a heat roller having an resilient layer such as a silicon rubber on the surface of a roller core bar is used.
When the heat roller is hard, the surface of the heat roller does not follow the undulation of the sheet or toner layer, and in a half tone image, the blackening manner is different depending on dots, and the image granularly appears, and in a solid portion, the uneven gloss appears, and the image quality is lowered. In a monochromatic image, the same image quality lowering is generated, but, because the line image is main, it is comparatively inconspicuous. Accordingly, for the improvement of the image quality of the color image, the fixing heat roller having an resilient layer is absolutely necessary.
In the image forming apparatus provided with the roller fixing type fixing apparatus composed of the heat roller whose heat conducting substrate (core bar) is covered by the resilient layer, and further which has a heat resistive releasing layer thereon, and the pressure roller. As the heat resistive releasing layer of the surface layer, fluorine resin, fluorine rubber, or silicon rubber is adopted. In the case of image forming apparatus which forms the image only on a single side of the sheet, it is necessary that the heat resistive releasing layer of the surface of the heat roller is selected under considering the image quality, the toner offset property, the wrapping property of the sheet around the heat roller, and the durability.
The heat resistive releasing layer of the conventional heating roller has high minute hardness, and prevent the generation of the roller flaws, and maintains the durability. However, when the minute hardness of the heat resistive releasing layer is increased, the deterioration of the fixed image is conspicuous, and not so good in the image quality. Further, when the minute hardness of the heat releasing layer is low, there is a trouble such as the generation of flows of the heat resistive releasing layer surface by a contact type temperature sensor or sheet separation claw.
For the hardness of the heat resistive releasing layer, the hardness (minute hardness) is regulated in Japanese Tokkai-2000-No. 75714, however, this regulates the minute hardness of the heat roller surface for the single side image formation onto the OHP sheet, and there is no solution of the problem in the case of the double side image formation as in the present invention.
Further, the heat roller described in Japanese Patent Tokkouhei No. 6-100876 is composed of 4 layer structure of a roller substrate, a silicon rubber layer, a composite layer formed of rubber and resin, and resin layer, and by the composite layer as an intermediate layer, the silicon rubber layer as the lower layer and the resin layer as the upper layer are strongly combined. For the heat roller formed of this 4 layer structure, there has been no regulation of the thickness or the hardness, and it can not serve for solving the problem at the double sides image formation as in the present invention.
The object of the present invention is to provide a fixing apparatus and an image forming apparatus which can solve the conventional problems described above.
The above described object is attained by the following structures.
(1) A fixing apparatus which heats and fixes a toner image formed on a transfer sheet by a heat rotating body in which a heat resistive resilient layer is covered on a heat conductive substrate and a heat resistive releasing layer is further formed thereon and which is heated by a heat source, and a pressure rotating body which pressure-contacts with the heat rotating body, wherein the minute hardness, measured by a micro hardness meter, of the heat resistive releasing layer is set to not lower than 55xc2x0 and not larger than 75xc2x0.
(2) A fixing apparatus which heats and fixes a toner image formed on a transfer sheet by a heat rotating body in which a heat resistive resilient layer is covered on a heat conductive substrate and a heat resistive releasing layer is further formed thereon and which is heated by a heat source, and a pressure rotating body which pressure-contacts with the heat rotating body, wherein the toner image is formed by using a polymerization toner produced by a polymerization method, and the softening point of the polymerization toner is not higher than 125xc2x0 C.
(3) An image forming apparatus which is characterized in that it is provided with the fixing apparatus described in (1) or (2), and an image forming means and a sheet conveying means.